Without limiting the scope of the invention, its background is described in connection with an integrated transmitter circuit, as an example.
Heretofore, in this field, advanced microwave systems have required different types of devices for the optimum operation of multifunction circuits. For example, the transmitter circuits of communication and radar systems are better addressed with the use of heterojunction bipolar transistors (HBTs) because of their higher efficiency and higher power density. On the other hand, receiver circuits of these same systems prefer the use of field effect transistors (FETs), such as high electron mobility transistors (HEMTs), to minimize the noise figure and therefore improve the receiver sensitivity. Other circuit functions in these systems, such as transmit/receive (T/R) switches and phase shifting functions may best be accomplished with PIN diodes.
High-speed three terminal devices such as HBTs and heterojunction FETs (HFETs) and microwave diodes such as PIN, IMPATT, mixer, etc. are normally fabricated by epitaxial growth techniques on insulating substrates. Because of vastly differing epitaxial layer properties for each one of these devices, each device structure is grown by an epitaxial growth technique that is best suited to the desired properties of that layer. Furthermore, the high-speed (or microwave) requirements placed on the device forces the growth of each device structure alone on the substrate to maximize performance. Integration of different types of microwave devices on the same substrate has been attempted in the past by epitaxial re-growth or ion implantation techniques (see, for example, U.S. patent application Ser. No. 07/560,501, assigned to the same assignee as the present case).
The integration of two or more heterojunction devices on the same substrate cannot be implemented by the method described above. Each heterojunction growth requires a separate epitaxial growth specifically designed to optimize the performance of each heterojunction type. The integration of an HBT and HEMT has been attempted in the past (see, for example, G. Sasaki et al., "Monolithic Integration of HEMTs and HBTs on an InP Substrate and Its Application to OEICs", Int. Electron Dev. Meeting Technical Digest, 1989, p. 896) by sequentially growing all of the epitaxial layers required for each device at one time, and subsequently removing any unwanted layers in the area of each device during fabrication. This method is called "stacked-layer method".
Some of the problems posed by the stacked-layer method have been a large number of layers required, the need to remove portions of several layers during processing and a resultant non-planar device. Accordingly, improvements which overcome any or all of the problems are presently desirable.